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WATER QUALITY AND USE: MANAGEMENT PRACTICES FOR FERTILIZATION
  MG Manual Reference
Ch. 16, pp. 5 - 7

[Management Practices For Fertilization: lawns | gardens | ornamentals ]

HOME LAWNSTop

Home lawns can be improved through the proper use of fertilizers. Excessive amounts or the wrong type of fertilizer will not provide an effective treatment for your lawn and may increase the risk of groundwater contamination. The following are some management practices that should be used to assure a healthy lawn and reduce the infiltration of lawn chemicals into the water supply.
Soil Testing - Soil testing can be helpful if your lawn is not performing as it should. Soil testing is costly for a home owner since home test kits are of little value for Arizona soil. To obtain an accurate soil test a soil sample must be sent to a certified soils laboratory. County Extension offices have a list of labs for your use. In general, soils in Arizona will be lacking in nitrogen and phosphate. Regular applications of these two elements should prevent the majority of problems due to nutrient deficiencies. To avoid over application of nitrogen it is best to apply small amounts of more frequently.
Fertilizer Types - Fertilizers are generally described by three numbers such as, 20-5-5, 16-4-8. These numbers refer to the percentage by weight of nitrogen, phosphorous, and potassium respectively, contained in the fertilizer. The nitrogen content in lawn fertilizers is usually higher than in garden fertilizers. Nitrogen promotes leaf formation and a deep color. Some phosphorous and potassium should accompany it to maintain a proper balance and to avoid over stimulation that might encourage disease. The amount of nitrogen that will be available to the lawn depends on whether the nitrogen in the fertilizer is quickly-available or slowly-available. Quickly-available nitrogen is water soluble and becomes available to plants immediately. Slowly-available nitrogen is released over a longer period of time. From a groundwater quality standpoint, quickly-available nitrogen is more prone to leaching and should be applied very carefully. The portion of the nitrogen in a bag of fertilizer that is slowly available is listed on the bag as Water Insoluble Nitrogen (WIN). For instance, a 20-5-5 fertilizer with a 5% WIN actually has 5/20 or 1/4 of the nitrogen in the slowly available form. Nitrogen in this form is more slowly available and releases over a longer period of time, and would be less likely to leach through the soil to the water table.
Amount, Frequency, and Timing of Fertilizer Application - The proper amount and scheduling of fertilizer application depends on a variety of factors including the source of nitrogen, soil type, type of turf grass, and whether or not clippings are collected. If the nitrogen in the fertilizer is slowly available, higher rates can be applied with less frequency than otherwise. Sandy soils are more prone to leaching than silt loam or clay loam soils. Higher levels or more frequent nitrogen applications may be required for sandy soils. However, great care should be exercised when applying fertilizers to sandy soils to reduce the chance of groundwater contamination. A slowly available nitrogen source may provide the best insurance against leaching in sandy soils while providing the proper nutrients as well. Cool-season grasses, such as Kentucky bluegrass, tall fescue, perennial ryegrass, and red fescue have a lower requirement for nitrogen than a warm-season grass like Bermuda grass. The timing of application is also important. Excessive spring application of nitrogen to cool-season grasses can be detrimental because it leads to excessive leaf growth at the expense of root development.
This makes the lawn more susceptible to summer disease and drought. Late summer and early fall applications of nitrogen to warm-season grasses encourage excessive fall growth and winter injury. Lawn clippings return a large amount of nitrogen and potassium to a lawn, and don't contribute significantly to thatch buildup. If clippings are removed, greater amounts of fertilizer will be required.

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